This invention relates generally to apparatus for optically measuring the concentration of a particulate in a fluid and to a method of linearizing the relationship between the particulate concentration and the signal developed by the apparatus over at least two orders of magnitude of concentration values.
Among the prior art of interest are the U.S. Pat. Nos. 3,420,609 to Kozawa; 3,713,743 to Simms; 3,724,957 to Tamate et al; and 4,047,815 to Sedlacek.
Concentrations of a particulate in a fluid, generally speaking, refers to the nature and the amount of discrete aggregations of material in the fluid which differ from the pure fluid itself. In case of liquids, and when optical properties are most important, the term "turbidity" is often used and is observed as the degradation of the contrast of an image transmitted through the liquid (for example the Jackson candle technique) or as a percentage of light emerging from the sample at angles different from the direct transmitted beam.
One measurement of concentration, which is in common use in the prior art, is to pass a beam of optical radiation, such as light, through the fluid, measure the intensity of the transmitted light and of light scattered at a preselected angle, and develop the ratio of the scattered light to the transmitted light. More particularly, the prior art method is to plot the ratio for two known concentrations on a graph and utilize the slope of the line (rate of change) passing through the two points as a function of the concentration. As long as the concentration of the particulate in the fluid has values which are less than say 200 parts per million (ppm), the assumption that the concentration varies linearly with ratio of the scattered to the transmitted light presents a fairly good approximately to the actual behavior of the relationship. At values greater than 200 ppm, the relationship becomes nonlinear.
The reasons for the nonlinearity of the relationship of the ratio to the concentration at higher values of particular concentration are not entirely understood, but the following factors contribute to it. When the concentration is low, most of the light is transmitted and very little of the light is scattered so that the scattered light is small and the transmitted light is large and the ratio would therefore be small. When the concentration increases, the scattered light also increases and diminishes the transmitted light. Also, the particulate will absorb light which is neither scattered nor transmitted and which will therefore not follow Beers law which gives the intensity of the transmitted light as a function of concentration.
While it is, of course, possible to plot the relationship between the ratio of scattered to direct light for known concentrations, and use that graph to determine any unknown concentrations, it is often desirable to develop a measurement signal that is linear with concentration. Such a linear relationship makes it more convenient to apply to meters, counters, controllers and the like.
One method heretofore utilized to linearize the relationship between concentration and a measured quantity is to apply the ratio signal (scattered over direct) to a linearizing network and utilize the output of such linearizing network. However, such networks are expensive to design and to manufacture, and require individual adjustment for each measuring apparatus.